This invention relates to a fabrication process for superconducting wires and coils, involving isostatic pressing, and heat treating steps which result in improved critical current density properties.
Perovskite related ceramic oxides, comprising alkaline earth metal-copper oxide, such as orthorhombic yttrium-barium-copper oxide materials, usually characterized as YBa.sub.2 Cu.sub.3 O.sub.7 or "1:2:3 ceramic oxides", are well-known "high temperature" superconductor materials. This 1:2:3 ceramic oxide material has been found to provide electrical superconductivity, that is, essentially no electrical resistance, at temperatures less than or in the region of 93 K.
The 1:2:3 ceramic oxides and other rare earth metal-alkaline earth metal-copper oxide based ceramics can operate in the superconducting mode near the 77 K boiling point of relatively inexpensive and plentiful liquid nitrogen, and could find increased use in composite windings for high current magnets, energy storage coils, long distance power transmission, and the like. However, 1:2:3 ceramic oxides and other superconducting ceramic oxides, generally made from sintered component oxide particles, are hard and brittle, and are not easily fabricated into fine wire or windings.
In U.S. Pat. No. 4,826,808 (Yurek et al.) a method of forming superconducting wires, rods and rings was taught, where pure metals, such as La, Ba, and Cu, which when oxidized would produce superconductors, preferably having silver, platinum, palladium or gold, added, were melted under vacuum and then melt spun to form the desired shape. Chill casting, centrifugal casting and the like were also taught to form the shape. Thermal-mechanical processing, such as wire drawing, extrusion, rolling and hot isostatic pressing, was employed to form a final, useful metal alloy shape. Then the alloy shape was oxidized, to form the alloy into the superconducting oxide, such as LaBa.sub.2 Cu.sub.3 O.sub.7, containing interdispersed noble metal which improved the ductility and strength of the shape.
In a less complicated method to solve brittleness problems, Jin et al., in Applied Physics Letters, "High T.sub.c Superconductors Composite Wire Fabrication," Vol. 51, No. 3, Jul. 20, 1987, pp 203-204, placed a metal cladding around an already formed, heat treated, 1:2:3 ceramic oxide powder superconducting core. The metal cladding, which was Ag, or Cu with a Ni/Au oxygen diffusion barrier, allowed ease of drawing into fine wire form, from 0.6 cm to 0.02 cm diameter, and also provided an alternate electrical conduction path in case the ceramic oxide lost its superconducting properties, that is, became normal or resistive. Ag was found particularly advantageous as a cladding. The drawn wires were then annealed at 900.degree. C. in oxygen. Multifilamentary ribbon composites were also formed.
U.S. Pat. No. 4,863,804 (Whitlow et al.) taught improved multifilamentary wire having less single filament distortion and improved dimensional uniformity. There, a sheath having a composite wall of ductile outer skin such as copper, and high strength inner wall, such as copper dispersion hardened with Al.sub.2 O.sub.3, was filled with already formed pellets of superconducting material, such as Nb.sub.3 Sn, Nb.sub.3 Al, NbC, and the like. It was then rolled or drawn to a reduced cross-section, annealed, and then cold extruded to from a wire having a hexagonal cross-section. Then, bundles of the hexagonal wires, along with pure high conductivity copper wires were placed in a copper sheath and the whole consolidated by hot isostatic pressing at 900.degree. C. for 2 hours at 1,057 kg/cm.sup.2 (15,000 psi). This consolidated wire bundle was then rolled or drawn to reduce the diameter and then cold extruded.
Already formed, round or hexagonal wires having silver sheaths containing YBa.sub.2 Cu.sub.3 O.sub.7 cores, can similarly be bundled and consolidated in a conducting tube of Cu, Ag, or Ag-Pd by hot isostatic pressing techniques, as taught by U.S. Pat. No. 5,017,553 (Whitlow et al.) filed on Jan. 25, 1990 (W.E. Case 55,279).
None of these processes, however, solved the unique problems associated with processing metals with ceramics. Silver sheathed, ceramic oxide superconducting wires, made, for example by the process of Jin et al., exhibit the following disadvantages: filament uniformity and electrical continuity are far from ideal, due to the inherent difficulties in co-processing a soft metal and a ceramic, and the metal sheathed ceramics are not resistant to damage from mechanical strain occurring in bending or forming operations. Also, ambient pressure heat treatments which involve melting of some core material result in core microstructures which are porous, due to gas evolution and the changes from the solid to the liquid and back to the solid state. This voidage within the core results in a reduced current density for the superconducting wire.
What is needed is a method of processing metal sheathed ceramics having superconducting capability, which solves problems of processing soft metal and ceramic, provides resistance to subsequent forming operations, and which provides a consolidated, void free, crack free ceramic core. It is a main object of this invention to provide such a process.